A new approach to isotopic analysis of carbon, oxygen, sulfur and nitrogen in microsamples has been established. Samples were conventionally prepared by mixing microsamples to be analyzed with reference samples with known δ values in a proper proportion. and then analyzed for their stable isotopes as those at ordinary levels. According to the equilibrium relationships before and after mixing, the δ values of unknown microsamples were calculated. The δ15 N of the atmosphere was estimated at zero by this approach. which is concordant with the internationally recommended value.